Extraction of Carbon Dioxide and Hydrogen From Seawater and Hydrocarbon Production Therefrom

ABSTRACT

Apparatus for seawater acidification including an ion exchange, cathode and anode electrode compartments and cation-permeable membranes that separate the electrode compartments from the ion exchange compartment. Means is provided for feeding seawater through the ion exchange compartment and for feeding a dissociable liquid media through the anode and cathode electrode compartments. A cathode is located in the cathode electrode compartment and an anode is located in the anode electrode compartment and a means for application of current to the cathode and anode is provided. A method for the acidification of seawater by subjecting the seawater to an ion exchange reaction to exchange H +  ions for Na +  ions. Carbon dioxide may be extracted from the acidified seawater. Optionally, the ion exchange reaction can be conducted under conditions which produce hydrogen as well as carbon dioxide. The carbon dioxide and hydrogen may be used to produce hydrocarbons.

This application claims priority to, and the benefit of, U.S.Provisional Application No. 61/333,553 filed on May 11, 2010, theentirety of which is herein incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to methods and apparatus for extractingcarbon dioxide and hydrogen from seawater and to processes forhydrocarbon production including the carbon dioxide extraction method.

2. Description of the Related Technology

It is desirable to be able to produce jet fuel at sea to supportaircraft carrier flight operations. In-theater, synthetic fuelproduction would offer significant logistical and operational advantagesby reducing dependence on increasingly expensive fossil fuels and byreducing the vulnerabilities resulting from unprotected fuel delivery atsea. A ship's ability to produce a significant fraction of the battlegroup's fuel for operations would increase the operational flexibilityand time on station by reducing the mean time between refueling.

Technologies currently exist to synthesize hydrocarbon fuel on land,given sufficient primary energy resources such as coal. Davis, B. H.Topics in Catalysis 2005, 32, 143-168. However, these technologies arenot CO₂ neutral, and they are not practical for sea-based operation.Extracting carbon dioxide from seawater is part of a larger project tocreate liquid hydrocarbon fuel at sea. Hardy, D. H., et al., “Extractionof Carbon Dioxide From Seawater by Ion Exchange Resin Part I: Using aStrong Acid Cation Exchange Resin,” Memorandum Report 6180-07-9044;Naval Research Laboratory: Washington D.C., Apr. 20, 2007; Willauer, H.D., et al., “Recovery of CO₂ from Aqueous Bicarbonate Using a GasPermeable Membrane,” Energy & Fuels, 2009, 23, 1770-1774; Willauer, H.D., et al., “Recovery of [CO₂]_(T) from Aqueous Bicarbonate Using a GasPermeable Membrane,” NRL Memorandum Report, 6180-08-9129, 25 Jun. 2008;Willauer, H. D., et al., “Extraction of CO₂ From Seawater By IonExchange Resin Part II: Using a Strong Base Anion Exchange Resins,” NRLMemorandum Report, 6180-09-9211, 29 Sep. 2009; Dorner, R. W., et al.,“Influence of Gas Feed Composition and Pressure on the CatalyticConversion of CO₂ Using a Traditional Cobalt-Based Fischer-TropschCatalyst,” Energy & Fuels, 2009, 23, 4190-4195; Dorner, R. W., et al.,“Effects of Loading and Doping on Iron-based CO₂ HydrogenationCatalyst,” NRL Memorandum Report, 6180-09-9200, 24 Aug. 2009; Dorner, R.W., et al., “Mn doped Iron-based CO₂ Hydrogenation Catalysts: Detectionof KAlH₄ as part of the catalyst's active phase,” Applied Catalysis A,January 2010, Vol. 373, Issues 1-2, pp. 112-121; and Willauer, H. D., etal., “The Effects of Pressure on the Recovery of CO₂ by Phase Transitionfrom a Seawater System by Means of Multi-layer Gas Permeable Membranes,”J. Phys. Chem. A, 2010, 114, 4003-4008. CO₂ as a carbon feedstock couldbe catalytically reacted with hydrogen to form diesel and/or jet fuel.The hydrogen could be produced through commercial off the shelfconventional electrolysis equipment, and the electrical energy for thisprocess would be derived through nuclear power or Ocean Thermal EnergyConversion (OTEC). Mohanasundaram, S. Renewable PowerGeneration-Utilising Thermal Energy From Oceans. Enviro. Sci. & Eng.2007, 4, 35. Avery, W. H.; Wu, C. Renewable Energy From The Ocean;Oxford University Press: New York, 1994. This synthetic fuel productionprocess could provide an alternative energy source to fossil fuels.

Practical, efficient, and economical methods of extracting largequantities of CO₂ from seawater must be developed before a sea-basedsynthetic fuel process that combines hydrogen produced by nuclear poweror solar OTEC with CO₂ to make jet fuel can be envisioned. The ocean'spH is kept relatively constant at approximately 7.8 by a complexcarbonate buffer system. 96% of the carbon in the oceans is in the formof HCO₃ ⁻. At pH of 4.5, 99% of all carbonate species in seawater existas H₂CO₃. Thus, in order to convert HCO₃ ⁻ to H₂CO₃, the pH of seawatermust be lowered.

CO₂ dissolved in water is in equilibrium with H₂CO₃ as shown in equation1 as:

CO₂+H₂O

H₂CO₃  (1)

The hydration equilibrium constant is 1.70×10⁻³. This indicates thatH₂CO₃ is not stable and gaseous CO₂ readily dissociates at pH of 4.5,allowing CO₂ to be easily removed by degassing once the seawater hasbeen acidified to ensure that the unstable H₂CO₃ is the predominantcarbonate species, as discussed above.

A detailed composition of seawater shows a carbon concentration of 28ppm (˜100 mg/L as CO₂). Assuming that the carbon exists as HCO₃ ⁻, theHCO₃ ⁻ concentration in seawater will be approximately 142 ppm (0.0023M), therefore approximately 23 mL of 0.100 M hydrochloric acid isrequired per liter of seawater:

HCl+HCO₃ ⁻→H₂CO₃+Cl⁻  (2)

CO₂ exists only in the dissolved gas form when the pH of seawater isdecreased to 6 or less Johnson, K. M., King, A. E., Sieburth, J.Coulometric TCO₂ Analyses for Marine Studies: An Introduction. MarineChem. 1985, 16, 61. A strong cation exchange resin material can be usedto acidify the seawater to below pH 6. Hardy, D. H. Zagrobelny, M.;Willauer, H. D.; Williams, F. W. Extraction of Carbon Dioxide FromSeawater by Ion Exchange Resin Part I: Using a Strong Acid CationExchange Resin; NRL Memorandum Report 6180-07-9044; Naval ResearchLaboratory Washington D.C., Apr. 20, 2007. However, the volume of waterper unit weight of resin required to regenerate the resin was muchlarger than the volume of CO₂ recovered, and potentially larger than thevolume of fuel produced from the CO₂. As a result the approach wasdeemed impractical for a sea-based application.

Thus as one further avenue to exploit the pH as a means to recovercarbon from the sea, an electrochemical acidification cell that is ableto decompose water into H⁺, OH⁻, hydrogen and oxygen gas by means ofelectrical energy has been developed and tested. The effects ofacidification cell configuration, seawater composition, flow rate, andcurrent on seawater pH are discussed. These data are used to determinethe feasibility of this approach for a carbon capture process.

The approach described within is considerably different than traditionalelectrolysis methods that are specifically tailored toward theproduction of hydrogen and oxygen from seawater and or fresh water. Mostmodern military submarines generate their breathing oxygen from theelectrolysis of fresh water. The difficulty with current seawaterelectrolysis technology for hydrogen production is the formation ofchlorine gas and thus electrodes are modified such that only oxygen isevolved at the anode. Kato, Z., et al., “Energy-Saving SeawaterElectrolysis for Hydrogen Production,” J. Solid State Electrochem.,2009, 13, 219-224.

SUMMARY OF THE INVENTION

In a first aspect, the present invention relates to an apparatus forseawater acidification. The seawater acidification apparatus includes anion exchange (IX) compartment, a cathode electrode compartment, an anodeelectrode compartment and cation-permeable membranes, which separate thecathode and anode electrode compartments from the ion exchangecompartment. The apparatus includes a means for feeding seawater throughthe ion exchange compartment and means for feeding a liquid mediacapable of dissociating to provide acidic ions through the anode andcathode electrode compartments. The device also includes a cathodelocated in the cathode electrode compartment, an anode located in theanode electrode compartment and a means for application of current toeach of the cathode and anode to create the driving force for the ionexchange process.

In a second aspect, the present invention relates to a method for theacidification of seawater. In this method, seawater is subjected to anion exchange reaction to exchange H⁺ ions for Na⁺ ions to therebyacidify the seawater.

In a third aspect, the present invention relates to a method forextracting carbon dioxide from seawater. In the method, seawater issubjected to an ion exchange reaction to acidify the seawater to a pH of6.5 or below by exchange of H⁺ ions for Na⁺ ions in the seawater. Oncethe seawater has been acidified, carbon dioxide is extracted as boundcarbon dioxide in the form of bicarbonate, or the acidified seawater isdegassed to obtain gaseous carbon dioxide. Optionally, the ion exchangereaction can be conducted under conditions which produce hydrogen aswell as carbon dioxide.

In a fourth aspect, the present invention relates to a method for theproduction of hydrocarbons from seawater. In the method, seawater issubjected to an ion exchange reaction to acidify the seawater to a pH of6.5 or below by exchange of H⁺ ions for Na⁺ ions in the seawater. Oncethe seawater has been acidified, the acidified seawater is degassed toobtain gaseous carbon dioxide. The carbon dioxide obtained by degassingis fed to a reactor with hydrogen to produce hydrocarbons. Optionally,the ion exchange reaction can be conducted under conditions whichproduce hydrogen as well as carbon dioxide and the hydrogen produced bythe ion exchange reaction can be used as a feed stream to thehydrocarbon production step.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of the total carbonate in seawater as expressed as afunction of pH

FIG. 2 shows an electrical chemical cell that lowers seawater pH bydecomposing water into H⁺ and OH⁻, hydrogen gas (H₂), and oxygen gas(O₂) by means of electrical energy.

FIG. 3 is a schematic representation showing the acidification apparatusemployed in Example 1 for the process of acidifying seawater.

FIG. 4 shows carbon dioxide removal as a function of seawater pH for theCO₂ Degassed Samples of Example 7 () and Example 8 (□).

FIG. 5 is a graph of the effluent seawater pH as a function of appliedcurrent for treatment of Key West Seawater in Example 5 (May 22, 2009)and Example 10 (Nov. 12, 2009).

FIG. 6 is a graph of electrical resistance as a function of time showingthe effect of scaling in the cathode compartment.

FIG. 7 is a schematic diagram of an apparatus for seawater acidificationand recovery of carbon dioxide and hydrogen in accordance with thepresent invention.

FIG. 8 is a schematic flow diagram of a carbon dioxide productionprocess in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

For illustrative purposes, the principles of the present invention aredescribed by referencing various exemplary embodiments. Although certainembodiments of the invention are specifically described herein, one ofordinary skill in the art will readily recognize that the sameprinciples are equally applicable to, and can be employed in othersystems and methods. Before explaining the disclosed embodiments of thepresent invention in detail, it is to be understood that the inventionis not limited in its application to the details of any particularembodiment shown. Additionally, the terminology used herein is for thepurpose of description and not of limitation. Furthermore, althoughcertain methods are described with reference to steps that are presentedherein in a certain order, in many instances, these steps may beperformed in any order as may be appreciated by one skilled in the art;the novel method is therefore not limited to the particular arrangementof steps disclosed herein.

It must be noted that as used herein and in the appended claims, thesingular forms “a”, “an”, and “the” include plural references unless thecontext clearly dictates otherwise. Furthermore, the terms “a” (or“an”), “one or more” and “at least one” can be used interchangeablyherein. The terms “comprising”, “including”, “having” and “constructedfrom” can also be used interchangeably.

In a first aspect, the present invention relates to an apparatus forseawater acidification. The major components of the seawateracidification apparatus 10 shown in FIG. 2 include a central ionexchange (IX) compartment 12, a cathode electrode compartment 14, ananode electrode compartment 16 and cation-permeable membranes 18, 19which separate the three compartments 12, 14, 16. Seawater is fed to ionexchange compartment 12 via a seawater inlet 20 and removed from ionexchange compartment 12 via a seawater outlet 22. A fluid capable ofdissociating to provide acidic ions, such as water, is fed to each ofthe cathode and anode electrode compartments 14, 16 via cathode fluidinlet 24 and anode fluid inlet 26. The fluid is removed from each of thecathode and anode electrode compartments 14, 16 via cathode fluid outlet28 and anode fluid outlet 30. The apparatus 10 also includes a cathode32 and an anode 34. Current is applied to each of the cathode 32 andanode 34 using any conventional electrode apparatus to create thedriving force for the ion exchange process.

The acidification apparatus 10 in FIG. 2 uses small quantities ofelectricity to exchange sodium ions for hydrogen ions in a centralseawater stream that is flowing adjacent to two cation exchangemembranes 18, 19. Seawater is passed through the central ion exchangecompartment 12 of the apparatus 10. Sodium ions are transferred throughthe membrane 18 closest to the cathode 32 and removed from the seawaterby means of direct current (DC) voltage. These sodium ions are replacedby hydrogen ions as the current drives the ions through the membrane 19closest to the anode 34. The hydrogen ions entering compartment 12 causeacidification of the seawater.

In apparatus 10, the anolyte is the water fed to the anode compartment16. At the anode 34, H⁺ is generated and it must migrate from thesurface of the anode 34, through the cation-permeable membrane 19, andinto the IX compartment 12 where it can replace Na⁺. Therefore theanolyte must be as dilute as possible such that the H⁺ are in excess anddo not compete with any other cations in the anolyte. Water with aconductivity of less than 20 μS/cm, such as reverse osmosis (RO)permeate, is preferably employed, though deionized water may also beused.

The anode and cathode may be made of any suitable material, based oncost, chemical stability, electrochemical performance characteristics,and the nature of the process involved. For example, the anode may bemade of a conventional material, such as ruthenium and/or iridium oxideon titanium metal, titanium oxide ceramic, and platinum plated titanium.Commercially available anodes of this type include those manufactured byEnglehard, Water Star (Newbury, Ohio), Eltech (Chardon, Ohio), andElectrode Products (Union, N.J.). The cathode may be stainless steel orsteel. Suitable materials are known to those skilled in the art andselection of a particular anode and cathode material is consideredwithin the skill of those knowledgeable in this field.

The cation membrane must selectively pass cations in preference toanions and may be manufactured of any suitable material, based on cost,chemical stability, electrochemical performance characteristics, and thenature of the process involved. Suitable materials are known to thoseskilled in the art and selection of a particular membrane material isconsidered within the skill of those knowledgeable in this field.Commercially available examples of heterogeneous and homogeneous cationmembranes that are useful in the present invention include, but are notlimited to, cation membranes manufactured or sold by Asahi Chemical,Dupont de Nemours, Membrane International Inc., Sybron/Ionics,Resintech, Ionpure, Hydro Components, Inc., Tulsion, Tokuyama Soda, MEGAas, and PCA-Polymerchemie Altmeier GmbH. Among these are the membranesformed of perfluorocarbon polymers having cation exchange functionalgroups that are resistance to oxidation and temperatures. Theconditioning and activation can be carried out according to themanufacturer's recommendations.

The use of cation exchange resins in the acidification modulecompartments can serve as an electro-active media that can exchange orabsorb sodium ions and release hydrogen ions. The hydrogen ionsgenerated at the anode thus regenerate the resin to the hydrogen form,releasing sodium ions to pass into the adjacent compartment. Theiremployment is particularly beneficial when feeding dilute solutions inthe electrode compartments as they help to lower the module's electricalresistance and increase efficiency.

Commercially available cation exchange resins that are useful in thepresent invention include, but are not limited to, cation resinsmanufactured or sold by Mitsubishi Chemical, Dow Chemical, Rohm and HaasCompany, Sybron Chemical Inc., Purolite, and Resin Tech Inc. Preferredcation exchange resins are synthetic organic strong acid cation exchangeresins that have sulfonated exchange sites as exemplified by standardcross-linked resins, such as IR-120 (Rohm and Haas), as well as highcross-linked resins, such as SK116 (Mitsubishi Chemical). High surfacearea macro-reticular or micro-porous type ion exchange resins havingsufficient ionic conductivity in the catalyst compartment are alsosuitable.

The catholyte is the water fed to the cathode compartment 14. Thecatholyte must be free of, or at least substantially free of hardnessions, such as calcium (Ca⁺²), ferrous (Fe⁺²) and magnesium (Mg⁺²). ThepH in the cathode compartment 14 is high enough to precipitate thesehardness ions. Therefore, a catholyte having a total concentration ofhardness ions less than 50 ppm, such as RO permeate or deionized watershould be employed.

Any suitable means may be employed to feed seawater to the ion exchangecompartment 12 and to feed liquid media to the cathode and anodecompartments 14, 16. Suitable means include pumps and pipes employinggravity feed.

Any suitable means may be employed for applying current to cathode andanode 32, 34. Suitable means include a power supply or other currentgenerating apparatus. Preferably, the means for applying charge to thecathode and anode 32, 34 is capable of reversing the polarity of thecathode and anode 32, 34 in order to regenerate the acidificationapparatus 10.

One skilled in the art will recognize that smaller particles result inincreased pressure drop through a compartment, which decreases operatingefficiency. Therefore, the maximum acceptable pressure drop limits theminimum size of the particle, which may vary depending on theacidification module design. The pressure drop through any of thecompartments in the acidification module is in the range of 0.1 to 100psi, and is typically between 1 to 10 psi. The size of the particle mustbe in the mesh size range of −1 inch to +60; even more preferably fromabout −¼ inch to +14; and most preferably from about −5 to +7.

The flow rate of solution through the ion exchange compartment is notcritical and can be selected from a broad range. However, the flow ratecan be controlled to produce a controlled seawater pH. The velocity ofthe solution through the ion exchange compartment should be at a levelwhere adequate agitation or stirring of seawater through the compartmentis achieved. A velocity between 10 to 500 cm/min; more preferably from30 to 210 cm/min; most preferably from 60 to 90 cm/min may be employed.

The process in accordance with this invention is operated at a currentdensity in the range of 5 to 200 mA/cm²; more preferably from about 20to 100 mA/cm²; and most preferably from about 50 to 70 mA/cm².

FIG. 3 shows an alternative apparatus 50 for the acidification ofseawater including a pump 202 for feeding seawater from a tank 201 tothe ion exchange compartment 12 of ion exchanger 208. Softened ordeionized water is fed from a tank 203 using a pump 205 to the cathodeand anode compartments 14, 16. A power supply 209 is provided to apply acurrent to the cathode and anode 32, 34 of ion exchange apparatus 208.Acidified sweater is obtained from the ion exchange compartment 12 and amixture of sodium hydroxide, oxygen and hydrogen is obtained from thecathode and anode compartments 14, 16.

Reactions for Electrochemical Acidification of Seawater

FIG. 2 shows an acidification apparatus 10 which can be employed toexchange Na⁺ for H⁺ in a stream that is flowing adjacent to twocation-permeable membranes 18, 19. A small amount of electricityfacilitates this exchange. Depicting seawater by sodium chloride (NaCl)and acidified seawater by HCl, the reactions within electrochemicalacidification apparatus 10 are as follows:

Anode: 2H₂O→4H⁺+O₂+4e ⁻  (3)

IX: 4|NaCl|_(Seawater)+4H⁺→4Na⁺+4|HCl|_(Acidic Seawater)  (4)

Cathode: 4H₂O+4Na⁺+4e ⁻→4NaOH+2H₂  (5)

Overall: 6H₂O+4NaCl→4|HCl|_(IX)+4|NaOH|+2|H₂|_(Cathode)+|O₂|Anode  (6)

The amount of H⁺ generated is proportional to the applied electricalcurrent, which follows Faraday's constant. Faraday's constant is definedas the amount of electricity associated with one mole of unit charge orelectron, having the value 96,487 ampere-second/equivalent.

For the anode reaction, 96,487 A sec will produce ¼ mole O₂ gas and 1mole H⁺ and for the cathode reaction, 96,487 A sec will produce ½ moleH₂ gas and 1 mole OH⁻. This allows calculation of the amount of H⁺, OH⁻,H₂, and O₂ produced per amp/second of current passed through theelectrodes:

Anode Reaction

$\begin{matrix}{{\left( \frac{{1/4}\mspace{14mu} {mole}\mspace{14mu} O_{2}}{96\text{,}487\mspace{14mu} A\text{-}\sec} \right)\left( \frac{60\mspace{14mu} \sec}{\min} \right)} = {0.000155\; \frac{{mole}\mspace{14mu} O_{2}}{A\text{-}\min}}} & (7) \\{{\left( \frac{1\mspace{14mu} {mole}\mspace{14mu} H^{+}}{96\text{,}487\mspace{14mu} A\text{-}\sec} \right)\left( \frac{60\mspace{11mu} \sec}{\min} \right)} = {0.000622\frac{{mole}\mspace{14mu} H^{+}}{A\text{-}\min}}} & (8)\end{matrix}$

Cathode Reaction

$\begin{matrix}{{\left( \frac{{1/2}\mspace{14mu} {mole}\mspace{14mu} H_{2}}{96\text{,}487\mspace{14mu} A\text{-}\sec} \right)\left( \frac{60\mspace{14mu} \sec}{\min} \right)} = {0.000311\frac{{mole}\mspace{14mu} H_{2}}{A\text{-}\min}}} & (9) \\{{\left( \frac{1\mspace{14mu} {mole}\mspace{14mu} {OH}^{-}}{96\text{,}487\mspace{14mu} A\text{-}\sec} \right)\left( \frac{60\mspace{14mu} \sec}{\min} \right)} = {0.000622\; \frac{{mole}\mspace{14mu} {OH}^{-}}{A\text{-}\min}}} & (10)\end{matrix}$

Therefore, for seawater with a HCO₃ ⁻ concentration of 142 ppm (0.0023M) and flow rate of 1 liter per minute, a theoretical applied current of3.75 A will be required to lower the pH to less than 6.0 and therebyconvert HCO₃ ⁻ to H₂CO₃.

$\begin{matrix}{\frac{\left( \frac{0.0023\mspace{14mu} {mole}\mspace{14mu} {HCO}_{3}^{-}}{Liter} \right)\left( \frac{1\mspace{14mu} {Liter}}{\min} \right)}{\left( \frac{0.000622\mspace{14mu} {mole}\mspace{14mu} H^{+}}{A\text{-}\min} \right)} = {3.57\mspace{14mu} A}} & (11)\end{matrix}$

Current efficiency can be defined as the ratio of the theoreticalminimum current predicted by Faraday's law to the actual current appliedto the electrodes of the acidification apparatus 10. In actuality,current efficiencies are never 100% and can range from 30 to 95% basedon the conductivity of the liquid being treated; the higher theconductivity, the greater the current efficiency. It is estimated thatcurrent efficiency for seawater is on the order of 70-90%. Therefore,

$\begin{matrix}{{{Actual}\mspace{14mu} {Current}} = {\left( \frac{{Theoretical}\mspace{14mu} {Current}}{{Current}\mspace{14mu} {Efficiency}} \right) = {\left( \frac{3.75\mspace{14mu} A}{80\%} \right) = {4.69\mspace{14mu} A}}}} & (12)\end{matrix}$

The theoretical amount of CO₂ that can be removed from the acidifiedseawater is 0.0023 moles per liter. Removal efficiency can be defined asthe ratio of the theoretical amount of CO₂ removed to the actual amountof CO₂ removed in the acidified seawater. Removal efficiencies are never100% and can range from 50-95% based on various unit operatingrequirements. The overall removal of CO₂ is conservatively estimated tobe approximately 50%.

$\begin{matrix}\begin{matrix}{{{Actual}\mspace{14mu} {Removal}} = \left( \frac{{Theoretical}\mspace{14mu} {Removal}}{{Removal}\mspace{14mu} {Efficiency}} \right)} \\{= \left( \frac{0.0023\mspace{14mu} {mole}\text{/}\min}{50\%} \right)} \\{= {0.0046\; \frac{{mole}\mspace{14mu} {CO}_{2}}{\min}}}\end{matrix} & (13)\end{matrix}$

The amount of H₂ gas generated at 4.75 A is

$\begin{matrix}{{\left( \frac{{1/2}\mspace{14mu} {mole}\mspace{14mu} H_{2}}{96\text{,}487\mspace{14mu} A\text{-}\sec} \right)\left( \frac{60\mspace{14mu} \sec}{\min} \right)\left( {4.75\mspace{14mu} A} \right)} = {0.0015\; \frac{{mole}\mspace{14mu} H_{2}}{\min}}} & (14)\end{matrix}$

Under these theoretical conditions, the molar ratio of H₂ and CO₂ is0.32. Increasing the current increases the molar ratio of hydrogen tocarbon dioxide with no effect on the operation of the acidificationapparatus 10. H⁺ generated will either exchange with Na⁺ in the seawaterto further lower its pH or migrate through the IX compartment 12 andinto the cathode compartment 14 where it will combine with OH⁻ to formwater.

Bound CO₂ can be captured from seawater in the form of bicarbonate. CO₂in seawater of pH of less than 4.5 can be naturally and completelydegassed upon exiting the acidification cell by exposure to theatmosphere. CO₂ in seawater having a pH greater than 4.5 may requireassistance to degas, by, for example, vacuum degassing for complete CO₂removal. The degree of vacuum degassing required to completely removeCO₂ increased as the seawater pH increased.

The cation exchange resin can be electrolytically regenerated, allowingsimultaneous and continuous ion exchange and regeneration to occurwithin the apparatus. This eliminates the need for regeneration bycaustic chemicals that are not ideal for a sea-based application. Thedegree of ion exchange and regeneration within the cell is a function ofthe current applied. Lowering the pH of seawater is an electricallydriven membrane process, where seawater pH is proportional to appliedcurrent and independent of the media contained within the IXcompartment. The examples below show that carbon dioxide was readilyremoved from seawater at pHs of less than 6.0. Unassisted and nearcomplete degassing was observed in seawater samples of pH of 5.0 andless. Assisted degassing by vacuum was required in seawater samples atpHs of greater than 5.0. The relationship between seawater bicarbonateconcentration, applied current, and seawater pH was demonstrated usingKey West seawater. The experimental set-up did not allow for recoveriesof greater than 95%. It is important to know the upper limit of recoverysince resources (space and energy) are required to produce a dilutewater stream for feeding to electrode compartments 24, 26.

In addition to carbon dioxide, the apparatus produced a portion of thehydrogen needed for a hydrocarbon synthesis process with no additionalenergy penalty. The production of hydrogen gas at the cathode as abyproduct occurred at a rate that correlated with the applied current.Thus the applied current to the apparatus can be increased to generatemore hydrogen gas with no negative performance or operational effects onthe acidification process. The acidification apparatus' ability toproduce a portion of the hydrogen needed for the downstream synthesis ofhydrocarbons from the recovered carbon dioxide reduces the operationalfootprint of the process, thus making the technology more feasible for asea-based application.

In another aspect, the present invention relates to a method for theacidification of seawater. In the method, seawater is subjected to anion exchange reaction to exchange H⁺ ions with Na⁺ ions in the seawaterto thereby acidify the seawater. The seawater is acidified to a pH ofless than about 6.5, more preferably, to a pH of less than about 6.0,even more preferably to a pH of less than about 5.0 and, mostpreferably, to a pH of less than about 4.5. Acidification of theseawater causes carbonic acid formation in the seawater, which allowsfor recovery of carbon dioxide from the acidified seawater.

Bound carbon dioxide can be removed from the acidified seawater in theform of bicarbonate. Alternatively, the acidified seawater can bedegassed to obtain carbon dioxide. Acidified seawater can be directlydegassed by exposure to atmosphere if the pH of the acidified seawateris less than about 5.0 and, greater recoveries of carbon dioxide can beobtained if the pH of the acidified seawater is less than about 4.5. Foracidified seawater having a pH of from about 4.5 to about 6.5, degassingmay be assisted by, for example, application of a vacuum. A suitableapparatus for degassing acidified seawater is a vacuum stripper. Anyother device that is capable of creating a vacuum such that the gaspressure is substantially less than atmospheric pressure may beemployed.

Referring to FIG. 7, there is shown an apparatus 200 for recovery ofcarbon dioxide and hydrogen from seawater. The apparatus 200 includes apump 202 which pumps raw seawater through a filter 204, preferablyhaving pores of about 5 microns to filter particulate matter from theseawater. The filter 204 is optional, though preferred to reduce thepotential for fouling, especially in embodiments employing seawaterreverse osmosis device 206. The filtered seawater may then be fed to theion exchanger 208, as shown. Alternatively, a portion of the filteredseawater may be fed to the seawater reverse osmosis device 206 and aportion of the filtered seawater may be fed to the ion exchanger 208. Ifa seawater reverse osmosis device is employed, concentrated seawater isdischarged as waste and permeate, dilute seawater, may be employed asthe liquid fed to the cathode and anode compartments 14, 16 of ionexchanger 208. Ion exchanger 208 is provided with a power supply 209with the capability of reversing the polarity of electrodes 32, 34 forregeneration of ion exchanger 208. Ion exchanger 208 functions as anacidification cell for acidification of the seawater fed to ion exchangecompartment 12 via seawater inlet 20 as described above in relation toFIG. 2.

Acidified seawater leaves ion exchange compartment 12 via seawateroutlet 22 and is then fed to vacuum stripper 210 to strip carbon dioxidegas from the acidified seawater. All or a portion of the fluid fromcathode and anode compartments 14, 16 may be fed to a vacuum stripper212 to extract hydrogen gas from the fluid. The process may provide bothcarbon dioxide and hydrogen gas, depending upon the specific conditionsemployed in ion exchanger 208.

While the plate-and-frame configuration of the acidification module 200illustrated in FIG. 7 is presently considered preferable, any moduleconfiguration can be used in accordance with this invention. Suchconfigurations include, but are not limited to, spiral and cylindricaltube designs. Module orientation, or positioning of the electrodes, maybe horizontal or vertical.

The thickness of the ion exchange compartment 12 can be varied dependingon the desired module performance. The ion exchange compartment 12thickness may be adjusted depending on operating parameters, such asflow rate, temperature, etc., and/or desired reactor performance, suchas product pH, electrical resistance, etc., and is typically between 0.3to 10.0 cm. The width of the chamber defining the ion exchangecompartment may be adjusted depending on operating parameters anddesired module performance, and the width to thickness ratio istypically between 5 and 20. There is no limit on the length of thechamber other than as dictated by practical construction and fluidpressure loss considerations.

Solution can be passed through any of the compartments in theelectrolytic reactor from bottom to top or from top to bottom. The flowdirection in two adjacent compartments can be co-current orcounter-current. Solution can be passed in parallel through eachcompartment, or a least one stream may flow in series through at leasttwo compartments.

The components of the acidification module 200 may be made from apolymeric (plastic) material that is chemically resistant andnon-conductive. Such materials include, but not limited to, PVC,chlorinated PVC, ABS, Kynar™, Teflon™, and other fluoropolymers.

The closing or sealing of the acidification module 200 can be achievedusing tie-bars, hydraulic or pneumatic type presses, or by solvent oradhesive bonding.

The method 300, shown in FIG. 8, involves the steps of providingseawater 302 and filtering seawater 304. Filtered seawater is fed to ionexchange compartment 12 of ion exchanger 208 and subjected to an ionexchange step 306 to acidify the seawater. A portion of the filteredseawater may be subjected to a seawater reverse osmosis step 308 and thepermeate of this step may be fed to the cathode and anode compartments14, 16 of ion exchanger 208 for use as the ion exchange fluid in ionexchange step 306. Acidified seawater produced in ion exchange step 306may then be fed to degassing step 310 to separate carbon dioxide fromthe acidified seawater. The ion exchange fluids from cathode and anodecompartments 14, 16 may be fed to stripping step 312 to separatehydrogen from the ion exchange fluids.

Carbon dioxide produced by the process of the present invention may befed to a reactor for the production of hydrocarbons such as jet fuels.Alternatively, the carbon dioxide may be used in industrial and marinefire extinguishing systems. Hydrogen produced by the process of thepresent invention may also be fed to the same hydrocarbon productionreactor or alternatively, may be used for other purposes, such as fuel.

Optionally, in the method of FIG. 8, the polarity of the electrodes 32,34 of the ion exchanger 208 may be reversed from time to time in ionexchange step 306 to regenerate ion exchanger 208.

As shown in the examples below, an electrochemical acidification cellhas been developed, tested, and found to be practical for recoveringlarge amounts of CO₂ from seawater for use as a carbon feedstock in asea-based fuel production process.

The invention will be illustrated by the following non-limitingexamples.

EXAMPLES Materials and Methods

In this test series two different acidification cells were evaluated;Nalco™ and Ionpure™. Tables 1 and 2 provide a detailed description ofeach cell's electrical and flow rate specifications along with thematerials used in the cell configurations. The anode used in the Nalco™cell is a dimensionally stable anode (DSA) composed of a mixed preciousmetal oxide coating on titanium. The Ionpure™ cell uses a platinumplated titanium anode.

This example demonstrates the flow rate to current ratio required tolower seawater pH to the target level. In addition to having differentanode materials, each cell contains a different type of cation-permeablemembrane. The Nalco™ cell contains Sybron's MC3470™ reinforced/castedcation permeable membrane, while the Ionpure™ cell contains apolyethylene extruded cation permeable membrane.

TABLE 1 Nalco ™ Cell Configured as an Electrochemical Acidification CellDimensions Approximate Overall Cell 14.0 cm × 36.5 cm × 6.0 cm DimensionIX Compartment Width 5.1 cm IX Compartment Height 30.1 cm IX CompartmentThickness 1.2 cm IX Compartment Volume 184.2 cm³ Membranes Active Area153.5 cm² Electrode Compartment Volume 98.4 cm³ Electrical SpecificationElectrode Active Area 153.5 cm² Max. Current Density 100 A m⁻² FlowSpecification Max. Flow Rate 20 cm³ s⁻¹ Max. Electrolyte Flow Rate 10cm³ s⁻¹ Max. Operating Temperature 60° C. Max. Operating Pressure 350kPa Materials Anode Dimensionally Stable Anode (DSA-600) Cathode 316LStainless Steel Membrane Sybron Cation-Permeable Membrane Molded Frameand End Block Acrylonitrile Butadiene Styrene (ABS)

TABLE 2 Ionpure ™ Cell Configured as an Electrochemical AcidificationCell Dimensions Approximate Overall Cell 33 cm × 61 cm × 8 cm DimensionIX Compartment Width 14 cm IX Compartment Height 35.5 cm IX CompartmentThickness 0.9 cm IX Compartment Volume 429 cm³ Membranes Active Area 497cm² Electrode Compartment Volume 214 cm³ Electrical SpecificationsElectrode Active Area 497 cm² Max. Current Density 400 A m⁻² FlowSpecifications Max. Flow Rate 35 cm³ s⁻¹ Max. Electrolyte Flow Rate 35cm³ s⁻¹ Max. Operating Temperature 60° C. Max. Operating Pressure 350kPa Materials Anode (Platonized Titanium) Cathode 316L Stainless SteelMembrane Ionpure Cation-Permeable Membrane Molded Frame and End BlockPolyethylene (PE)

Electrochemical Acidification Tests

FIG. 3 is a schematic representation of the acidification experimentalset-up. Seawater was passed upwardly through the ion exchange (IX)compartment. Deionized water at a pH of approximately 6.7 was passed, inseries, upwardly through the anode compartment and then upwardly throughthe cathode compartment. A controlled current was applied to the anodeand cathode in order to lower the pH of the seawater to the targetlevel. Only a single pass of seawater and deionized water through thecell was employed.

Five different types of seawater were evaluated in this test series.Four of these types were synthetic seawater formulations (designated asIO-1, IO-2, IO-3, IO-4) created from instant ocean seawater while theother was actual seawater taken from a laboratory in Key West Fla. (KW).The synthetic formulations were prepared at least 16 hours prior to useto allow the buffer salts to reach equilibrium. The concentrations andmeasured pH of the seawater solutions is given below. The pH of thesolutions varied as a result of equilibration with CO₂ gas in theatmosphere. These pH changes had no effect on the overall performance ofthe acidification cell. All pH measurements were conducted with astandardized Fisher combination glass electrode. The carbon dioxidecontent of the solutions was measured by a UIC Coulometric system (UICInc, Joliet, Ill.).

IO-1—Synthetic seawater was prepared by dissolving 41.1 gL⁻¹ of InstantOcean Sea Salt in deionized water at a total volume of 100 liters. ThepH of the synthetic seawater was 9.1±0.5. The CO₂ content was notmeasured.

IO-2—Synthetic seawater was prepared by dissolving 35 gL⁻¹ of InstantOcean Sea Salt in deionized water at a total volume of 140 liters. ThepH of the synthetic seawater was 8.4±0.2. The CO₂ content was notmeasured.

IO-3—Synthetic seawater, made using Instant Ocean Sea Salt at 35 gL⁻¹,was supplemented with 90 ppm of sodium bicarbonate and diluted withdeionized water to a total volume of 140 liters. The pH of the syntheticseawater was 8.2. The CO₂ content was measured to be approximately 100ppm.

IO-4—Synthetic seawater, made using Instant Ocean Sea Salt at 35 gL⁻¹,was supplemented with 90 ppm of sodium bicarbonate and diluted withdeionized water to a total volume of 140 liters. The pH was adjusted to7.6 using approximately 20 mLs of diluted hydrochloric acid (5 mL ofconcentrated HCl diluted to 50 mL) added to 100 liters of syntheticseawater. The CO₂ content was measured to be approximately 100 ppm.

KW—Actual Seawater from the Naval Research Laboratory Key West, Fla. ThepH was 7.6±0.2 and the CO₂ content was measured to be approximately 100ppm.

Degassing measurements were made on selective samples during the courseof the experiment. For each measurement carbon dioxide was degassed fromsolution using a Brinkmann Roto-Evaporator. A 20 mL sample was placed ina 1000 mL round bottomed flask and rotated at an rpm setting of 8 forfive minutes. A water aspirator was used to provide a vacuum ofapproximately 15 mm Hg.

Example 1

In the first example, the Nalco™ cell (Table 1) was configured such thata strong acid cation exchange resin was used to fill both the ionexchange compartment (IX) and the electrode compartments shown in FIG.2. IO-1 seawater was used as the influent, and passed through the IXcompartment at 140 ml/min for 165 minutes. The flow rate of thedeionized water passing through the electrode compartments was increasedto a final flow rate of 140 mL/min. The results summarized in Table 3show that when 3 amps of current where applied to the cell, the pH ofthe effluent seawater was lowered to between 2.08 and 2.15. As thecurrent was decreased from 3 to 0.25 amps, the pH of the effluentseawater was increased to 5.88. These results demonstrate that the pH ofthe effluent seawater can be lowered using the Nalco™ cell. Theseresults also show that the pH of the seawater is proportional to theapplied current and demonstrate that the cation exchange resin can beelectrolytically regenerated.

The flow rate to current ratio when the seawater pH was 5.88 isestimated to be 560 as follows: 140 mL/min/0.25 amps=560. This ratio ishigher than the theoretical ratio of 267 calculated for the sameconditions as described in equation 11 as:

1000 mL/min/3.75 amps=140 mL/min/X

X=0.525 Amps

Theoretical Ratio: 140 mL/min/0.525 amps=267.

This indicates that the HCO₃ ⁻ levels in the IO-1 synthetic formulationmay not be at the correct concentration, even though the pH was high.The recovery was 50% and was not lowered in this experiment. The term“recovery” is used to define the ratio of product quantity (influentseawater flow rate, Table 3) to the total feed quantity to the cell(influent seawater flow rate and influent deionized flow rate, Table 3),and is expressed as a percentage. The water required in the electrodecompartments must be dilute and free from ions that create hardness. Afiltration process such as reverse osmosis should be used to treat thistype of water. A high recovery allows the size of the filtration unit tobe minimized and the energy requirements for the unit to be reduced.

TABLE 3 Acidification of IO-1 seawater at 140 mL/min Effluent EffluentInfluent Seawater Influent DI Acidified DI Time, Flow Rate, Flow Rate,Seawater Cathode min Amp/Volt mL min⁻¹ mL min⁻¹ pH pH 15 3.0/9.0 150 802.08 12.69 30  3.0/10.0 140 77 — — 45  3.0/10.0 140 75 2.15 12.63 60 3.0/11.0 140 75 2.10 12.49 75  3.0/12.0 140 144 — — 90 2.0/9.0 140 1402.19 12.13 105 1.0/6.0 140 140 2.70 12.00 120 0.5/4.0 140 140 3.61 11.41135 0.25/3.0  140 140 5.88 11.26 150 0.0/0.0 140 140 — — 165 0.0/0.0 140140 8.76 10.41

Example 2

In the second example, O-2 was used to determine the effect that flowrate through the IX compartment had on the performance of the Nalco™cell. Initially IO-2 was pumped at 900 mL/min before a flow of 860mL/min was established and maintained after 90 minutes. The resultssummarized in Table 4 establish that at higher flow rates the pH of theeffluent seawater can be lowered using the acidification cell. Higherinfluent seawater flow rates increase the percent recovery from 50% to86%. In addition the results indicate that the flow rate to currentratio was improved from 560 to 716 (860 mL/min/1.20 amps).

TABLE 4 Acidification of IO-2 seawater at 900 mL/min Effluent EffluentInfluent Seawater Influent DI Acidified DI Time, Flow Rate, Flow Rate,Seawater Cathode min Amp/Volt mL min⁻¹ mL min⁻¹ pH pH 15 1.0/5.0 900 1407.31 11.79 30 1.5/7.0 900 140 6.13 12.06 45 1.5/7.0 864 140 5.95 12.0460 1.5/7.0 870 140 4.63 11.90 75 1.5/7.0 864 140 4.41 11.89 90 1.5/7.0868 140 4.30 11.75 105 1.5/7.0 860 140 4.18 11.76 120 1.5/7.0 860 1404.05 11.74 135 1.0/6.0 860 140 6.31 11.12 150 1.0/6.0 860 140 6.20 11.42165 1.0/6.0 860 140 6.27 11.52 180 1.2/6.5 860 140 6.17 11.20

Example 3

The third example examines the effects of replacing the strong cationexchange material in the IX compartment of the cell with inert ceramicparticles. The affects of the ceramic media on the ion exchangeefficiency of Na⁺ for H⁺ ions in the cell is determined in the resultsshown in Table 5.

TABLE 5 Acidification of IO-2 Seawater at 680 mL/min using InertMaterial Effluent Effluent Influent Seawater Influent DI Acidified DITime, Flow Rate, Flow Rate, Seawater Cathode min Amp/Volt mL min⁻¹ mLmin⁻¹ pH pH 15 1.0/8.0 680 140 8.29 11.88 30 1.0/8.0 680 140 8.48 11.8845 1.0/8.0 680 140 8.28 11.87 60 1.0/8.0 680 140 7.13 11.87 75 1.0/8.0680 140 6.62 11.90 90 1.0/8.0 680 140 6.10 11.88 105 1.0/8.0 680 1405.51 11.88 120 1.0/8.0 680 140 4.22 11.52 135 1.0/8.0 680 140 6.46 11.25150 0.8/6.0 680 140 6.30 11.33 165 0.9/6.5 680 140 5.87 11.36 1800.9/7.0 680 140 5.83 11.42

Example 4

At a flow rate of 680 mL/min to the IX compartment and an appliedcurrent of 1.0 amps, the pH of IO-2 is reduced from 8.29 to 6.3 in 150minutes. Since the pH of the IO-2 water can be lowered with inert mediain the IX compartment the process is considered to be an electricallydriven membrane process. The progressive decrease in the IO-2 pHindicates that the cation exchange resin in the anode compartment wasregenerating. Since the cation exchange resin in the anode compartmentwas in the sodium form during the start of the experiment and that H⁺ions from the oxidation of water on the anode exchanged on the resin andreleased Na⁺ ions. These Na⁺ ions then migrated through the cationexchange membrane and into the IX compartment. Breakthrough of H⁺ ionsinto the IX compartment began at 60 minutes when the pH of the seawaterbegan to decrease. See FIG. 1 for illustrative details. The high andstable pH of the NaOH is a result of this. The flow rate to currentratio was further improved from 716 to 755 (680 mL/min/0.90 amps) at arecovery of 83% using this process.

When the flow rates to the IX compartment and the electrode compartmentswere dropped (680 mL/min to 140 mL/min in the IX compartment) (120mL/min to 18 to 10 mL/min) as shown in Table 6, there was an improvementin the flow rate to current ratio 1,207 (140 mL/min/0.116 amps). Inaddition, the lower flow rates to the cell increased the recovery from88% to 95%.

TABLE 6 Acidification of IO-2 seawater at 140 mL/min using InertMaterial Effluent Effluent Influent Seawater Influent DI Acidified DITime, Flow Rate, Flow Rate, Seawater Cathode min Amp/Volt mL min⁻¹ mLmin⁻¹ pH pH 60 0.178/4.75 140 18 3.96 10.82 120 0.169/4.63 140 16 4.1910.92 180 0.136/4.27 140 17 5.30 10.84 240 0.116/4.03 140 18 6.07 10.82300 0.116/3.99 140 17 5.75 10.92 360 0.116/3.91 140 10 5.92 10.84 4200.116/3.79 140 10 5.88 10.88

Example 5

With the optimum cell configuration and flow rates established byprevious experiments, Key West seawater was used in order to simulatesample conditions that would be encountered in an actual ocean processfor sequestering CO₂. The results summarized in Table 7 show that theflow rate to current ratio was significantly reduced to approximately298 (140 mL/min/0.47 amps).

TABLE 7 Acidification of KW seawater at 140 mL/min using Inert MaterialEffluent Influent Seawater Influent DI Acidified Time, Flow Rate, FlowRate, Seawater min Amp/Volt mL min⁻¹ mL min⁻¹ pH 1 0.15/3.14 140 10 5.522 0.15/3.13 140 10 6.05 3 0.15/3.14 140 10 6.14 4 0.15/3.14 140 10 6.235 0.15/3.14 140 10 6.29 6 0.17/3.21 140 10 6.35 7 0.17/3.23 140 10 6.359 0.19/3.37 140 10 6.42 11 0.21/3.45 140 10 6.42 13 0.26/3.61 140 106.37 15 0.31/3.84 140 10 — 17 0.40/4.06 140 10 6.20 19 0.47/4.33 140 106.09 21 0.47/4.34 140 10 6.04 23 0.47/4.33 140 10 5.97

Example 6

This ratio is very similar to the theoretical ratio of 267, indicatingthe correct HCO₃ ⁻ level, and the recovery was at 94%. When the CO₂content of the KW seawater was measured by coulometry, it was found tocontain 4 times more alkalinity than measured in the IO-1 and IO-2seawater formulations. This is clearly evident from the data in Table 7which shows that it required 0.47 amps to decrease the pH of the KWseawater from 7.60 to 5.97. Previous acid titrations used to assess theCO₂ concentration of the KW seawater were consistent with the results ofthis example. An average of 20 mLs of 2.00E-⁰³ M of hydrochloric acidwas required to reduce the pH in a 20 mL sample of Key West seawater to6.0. This required 4.05E-⁰⁵ moles of hydrogen ions. Based on a flow rateof 140 mL/min and the use of Faraday's constant, 0.000622 mol H⁺/A-min(equation 9), a current of 0.46 amps will generate 4.05E-⁰⁵ moles ofhydrogen ions. The current efficiency in the electrolytic cell wasapproximately 98%.

In the sixth example, a new IO formulation (IO-3) was made that had asimilar alkalinity concentration as that found in KW seawater. However,the pH was 8.19 compared to the 7.67 measured for the KW seawater. Table8 shows that the amount of current required to reduce the pH of IO-3from 8.19 to 6.0 was approximately three times higher than that seen inprevious example. The flow rate to current ratio was approximately 111(140 mL min⁻¹/1.26 amps), which is lower than the theoretical ratio of267. The recovery was between 88% and 95%. The CO₂ content in IO-3 wasfound to be adequate so the low ratio may be attributed to theacidification cell not having been operated long enough to reachequilibrium.

TABLE 8 Acidification of IO-3 seawater at 140 mL/min using InertMaterial Effluent Influent Seawater Influent DI Acidified Time, FlowRate, Flow Rate, Seawater min Amp/Volt mL min⁻¹ mL min⁻¹ pH 15 0.46/4.58140 10 6.52 30 0.48/4.89 138 9 6.42 45 0.49/4.71 140 18 6.48 600.49/4.69 138 9 6.78 75 0.51/4.79 140 9 6.65 90 0.65/5.31 139 8 6.48 1050.72/5.66 140 18 6.36 120 0.99/6.74 140 18 6.24 135 1.12/7.27 140 196.10 150 1.26/7.90 140 19 5.94 165 1.44/8.46 140 19 5.78 180 1.66/9.75140 18 5.57 195  1.99/12.17 140 18 5.14 225  1.99/12.06 140 9 5.03 2551.31/9.77 140 9 5.86

Example 7

A new IO formulation for example 7 was made in an effort to create asynthetic system that had a similar pH and alkalinity to KW seawater.When IO-4 was used to challenge the acidification cell, the resultsshown in Table 9 indicate that the amount of current required to reducethe pH to 6.0 was approximately two times higher (0.47 vs 0.89 amps).The flow rate to current ratio was approximately 157 (140 mL/min/0.89amps), which is lower than the theoretical ratio of 267. The recovery atthese flow rates was between 88% and 95%. The CO₂ content in IO-4 wasfound to be adequate so the low ratio may be attributed to theacidification cell not having been operated long enough to reachequilibrium.

TABLE 9 Acidification of IO-4 seawater at 140 mL/min using InertMaterial Effluent Influent Seawater Influent DI Acidified Time, FlowRate, Flow Rate, Seawater min Amp/Volt mL min⁻¹ mL min⁻¹ pH 151.33/12.23 140 9 3.42 30 0.97/10.51 140 9 5.57 45 0.88/9.89  140 9 6.1175 0.89/10.20 140 9 6.03 105 0.89/10.50 140 9 6.09 135 0.89/10.88 140 196.19 210 1.51/13.79 140 19 2.76 255 1.15/12.43 140 19 4.42

Example 8

Demonstrating the feasibility of scaling this process was the objectiveof Example 8 (Table 10). The larger Ionpure™ cell (Table 2) wasconfigured similarly to the Nalco™ cell used in Examples 3-7. IO-4 wasused to challenge the cell at a flow rate of 1050 mL/min to the IXcompartment. From Table 10 the amount of current required to reduce thepH from 7.6 to 4.5 was approximately 4.87 amps. The flow rate to currentratio was approximately 216 (1050 mL min⁻¹/4.87 amps). This ratio islower than the theoretical ratio of 267. The recovery was 90%. Since theCO₂ content in IO-4, was similar to that measured in KW seawater, thelow ratio may be attributed the acidification cell not having beenoperated long enough to reach equilibrium.

TABLE 10 Acidification of IO-4 seawater at 1050 mL/min using InertMaterial Effluent Influent Seawater Influent DI Acidified Time, FlowRate, Flow Rate, Seawater min Amp/Volt mL min⁻¹ mL min⁻¹ pH 20 3.51/5.621050 116 7.18 40 4.49/6.74 1050 118 6.82 60 5.54/7.52 1050 118 6.24 806.53/8.40 1050 114 4.23 100 6.24/8.24 1050 114 3.44 120 5.02/7.42 1050116 4.75 140 4.75/7.24 1050 116 4.21

Example 9

The Ionpure™ cell was re-configured in Example 9 (Table 11) such that nomaterial was used in IX compartment of the cell. Table 11 shows that theresults were consistent with the results of Example 8 (Table 10) duringthe first 100 minutes of operation which further indicates that thisprocess is an electrically driven membrane process, whereby the mediacontained in the IX compartment has no effect on the acidification ofthe seawater. After 100 minutes of operation, voltage and flow rateirregularities occurred indicating internal disruption due tonon-supported membranes. The pH began to increase and the flow ratebegan to decrease in the IX compartment while the flow rate increasedand pH decreased in the electrode compartments, indicating a gross leakbetween compartments.

Towards the end of this Example 9 (Table 11), a crude electrode gas (H₂and O₂) capture experiment was performed, but both gases were capturedtogether due to a common electrode compartment inlet and outlet. Thecombined gas captured was approximately 10 mL/min per amp. This is closeto theoretical according to Faraday's law which indicated an expectedvalue of 10.5 ml/min (7.0 mL/min H₂+3.5 mL/min O₂) per amp at thestandard conditions of temperature and pressure.

TABLE 11 Acidification of IO-4 seawater at 1050 mL/min Without InertMaterial Effluent Influent Seawater Influent DI Acidified Time, FlowRate, Flow Rate, Seawater min Amp/Volt mL min⁻¹ mL min⁻¹ pH 20 4.70/9.401020 120 7.59 40 4.81/9.32 1040 116 7.17 60 4.97/9.37 1040 116 6.76 806.08/9.02 1020 118 6.31 100 6.07/8.42 1020 118 6.12 120 6.04/8.38 1000108 6.15

The pH of the effluent acidified seawater was measured for each ofExamples 1-9. In Examples 7 and 8, the effluent acidified seawater wascollected and 20 mL aliquots were placed in a 1000 mL round bottom flaskand degassed using a Brinkmann Roto-Evaporator. A water aspiratorprovided a vacuum of approximately 15 mm Hg. The carbon dioxide contentof each solution was measured by coulometry and plotted as a function ofpH, as shown in FIG. 4. In both Example 7 and Example 8, greater than98% carbon dioxide removal was achieved at pHs lower than 4.5. CO₂dissolved in water is in equilibrium with H₂CO₃. The hydrationequilibrium constant (1.70×10⁻³) indicates that H₂CO₃ is not stable andgaseous CO₂ readily dissociates at pH of 4.5, allowing CO₂ to be easilyremoved by degassing. This was observed during all of the foregoingexamples.

CO₂ in seawater samples of pH of less than 4.5 was naturally andcompletely degassed upon exiting the acidification cell (exposed toatmosphere during sampling). CO₂ content and pH were measured before andafter vacuum degassing and there was no significant difference in thesetwo measurements.

CO₂ in seawater samples having a pH greater than 4.5, requiredassistance by vacuum degassing for complete CO₂ removal. In thesesamples, CO₂ content and pH were measured before and after vacuumdegassing and there were significant differences in both measurements;CO₂ content decreased and pH increased. Although not quantified, itappeared that the degree of vacuum degassing required to completelyremove the CO₂ increased as seawater pH increased.

Example 10

Example 10 was conducted to confirm the results of acidification of KWseawater obtained in Example 5. The process was also scaled up by fivetimes to determine if the process can be linearly scaled up.

The acidification process of Example 5 was continued and operated for anadditional 105 minutes in this Example 10.

TABLE 12 Results from Example 10 Effluent Influent Seawater Influent DIAcidified Time, Flow Rate, Flow Rate, Seawater min Amp/Volt mL min⁻¹ mLmin⁻¹ pH 15 0.49/4.61 140 10 5.62 30 0.49/4.62 140 10 5.71 45 0.59/4.99140 10 5.49 60 0.68/5.39 140 10 5.15 75 0.79/5.77 140 10 4.37 900.73/5.53 140 10 4.68 105 0.75/5.66 140 10 4.32

The results of Example 10 (Nov. 12, 2009) were consistent with theresults of Example 5 (May 22, 2009), while keeping all variables andoperating conditions identical.

Example 11 Process Scale Up

A theoretical flow rate to current ratio was used to evaluate theexperimental results. In Example 5 KW seawater had a ratio of 298 withthe Nalco™ module (140 mL/min required 0.47 amps to lower the pH tobelow 6.00). This ratio will be used to determine if the process can bescaled up in a linear fashion.

The dimensions of the IX compartment were 0.050 m×0.301 m×0.120 m,giving an area of 0.015 m² and a volume of 0.001 m³. The Nalco™ modulewas operated at 140 mL/min or 0.037 gpm, giving the following tworatios:

Flow Rate to Area Ratio (gpm/m²)=2.4

Flow Rate to Volume Ratio (gpm/m³)=201

Example 11 was conducted at a higher flow rate to demonstrate that thescale up is linear. The same Nalco™ module was operated at five timesmore flow rate (700, mL/min) using KW seawater. The flow rate to currentratio was 298 (700 mL/min required 2.35 amps to lower the pH to 5.34).The two flow rate ratios were:

Flow Rate to Area Ratio (gpm/m²)=12.0

Flow Rate to Volume Ratio (gpm/m³)=1004

A large increase in flow rate to volume ratio did not affect performanceindicating that this ratio may be further increased. This provides agreat benefit since a marginal increase in module size will accommodatea significant increase in flow rate. However, increasing flow rate willincrease pressure drop through the IX compartment. The particle size ofthe inert media can be increased from, for example, 20 to 40 mesh (0.41to 0.76 mm) to 7 to 14 mesh (1.52 to 2.79 mm) in order to reduce thepressure drop.

Hardness Scaling

Hardness contained in seawater includes calcium (Ca⁺²) and magnesium(Mg⁺²) ions, and their total ion concentration is typically less than2,000 mgL⁻¹. Hardness ions can migrate from the ion exchange (IX)compartment to the cathode compartment and can be introduced into thecathode compartment by the water feeding the cathode compartment. In theforegoing examples, deionized water was used as the feed water to thecathode compartment, so the only hardness ions entering the cathodecompartment were from the IX compartment. It was initially assumed thatthe likelihood of hardness ions migrating from the IX compartment to thecathode compartment was negligible for the following three reasons:

-   -   The amount of cations that need to be exchanged to lower the pH        is less than 0.5% of the total cations present in the seawater.    -   The molar ratio of Na⁺ to hardness ions (Ca²⁺ and Mg²⁺) is        approximately 7 to 1.    -   The mobility coefficient favors Na⁺ (50.1 cm²Ω⁻¹eq⁻⁻¹) over        ½Ca²⁺ (59.5 cm²Ω⁻¹ eq⁻¹) and ½Mg²⁺ (53.0 cm²Ω⁻¹eq⁻¹).        However, the actual results indicated that there was an effect        as shown in FIG. 6.

FIG. 6 is a plot of electrical resistance (voltage divide by amperage)versus time. The signs of scaling are visible as a progressive increasein the electrical resistance over time. There was a 58.5% increase inelectrical resistance (from 4.07 to 6.45 Ohms) after 150 minutes ofoperation. Reversing the polarity of the electrodes every 15 to 20minutes can reduce fouling. This change in polarity causes scale andorganics to disassociate from the electrode surface. The flow will alsohave to switch during every polarity reversal. This can be accomplishedusing solenoid valves that are synchronized to the polarity reversal.This is a common feature that is used in the Electrodialysis Reversal(EDR) process to desalinate brackish water.

It is to be understood that even though numerous characteristics andadvantages of the present invention have been set forth in the foregoingdescription, together with details of the structure and function of theinvention, the disclosure is illustrative only, and changes may be madein detail, especially in matters of shape, size and arrangement of partswithin the principles of the invention to the full extent indicated bythe broad general meaning of the terms in which the appended claims areexpressed.

1. Apparatus for treatment of seawater comprising: an ion exchangecompartment, a cathode electrode compartment including a cathode, ananode electrode compartment including an anode, cation-permeablemembranes separating the cathode and anode electrode compartments fromthe ion exchange compartment, means for feeding seawater through the ionexchange compartment, means for feeding a liquid media capable ofdissociating to the anode and cathode electrode compartments, and meansfor application of a current to the cathode and anode to create thedriving force for the ion exchange process.
 2. The apparatus as claimedin claim 1, wherein the means for application of current appliessufficient current the cathode and anode to acidify the seawater to a pHof about 6.5 or less.
 3. The apparatus as claimed in claim 2, wherein acurrent density resulting from applied current is from about 5 to about200 mA/cm².
 4. The apparatus as claimed in claim 3, wherein the meansfor feeding seawater to the ion exchange compartment feeds the seawaterat a velocity of from about 20 to about 500 cm/min.
 5. The apparatus asclaimed in claim 4, wherein the means for applying current appliessufficient current to generate excess hydrogen in the anode and cathodecompartments.
 6. The apparatus as claimed in claim 1, wherein the meansfor applying current is capable of reversing the polarity of the anodeand cathode to regenerate the apparatus.
 7. The apparatus as claimed inclaim 1, wherein the means for application of current applies sufficientcurrent the cathode and anode to acidify the seawater to a pH of about4.5 or less.
 8. The apparatus as claimed in claim 1, further comprisinga device for separating carbon dioxide from acidified seawater obtainedfrom the ion exchange compartment.
 9. A method for treatment of seawatercomprising the step of subjecting the seawater to an ion exchangereaction to exchange H⁺ ions for Na⁺ ions in the seawater underconditions sufficient to lower a pH of the seawater to less than about6.5.
 10. The method as claimed in claim 9, wherein the conditions aresufficient to lower a pH of the seawater to less than about 4.5.
 11. Themethod as claimed in claim 9, wherein a current density resulting fromapplied current is from about 5 to about 200 mA/cm².
 12. The method asclaimed in claim 9, wherein the seawater is fed to the ion exchangereaction at a at a velocity of from about 20 to about 500 cm/min. 13.The method as claimed in claim 9, wherein sufficient current is appliedin the ion exchange reaction to generate excess hydrogen.
 14. The methodas claimed in claim 9, further comprising the step of reversing thepolarity of the anode and cathode.
 15. The method as claimed in claim 9,further comprising the step of separating carbon dioxide from acidifiedseawater obtained from the ion exchange reaction.
 16. The method ofclaim 15, further comprising the step of separating hydrogen from aliquid ion exchange media obtained from the ion exchange reaction. 17.The method of claim 15, further comprising the step of producinghydrocarbons from the carbon dioxide obtained in the carbon dioxideseparation step.
 18. The method of claim 16, further comprising the stepof producing hydrocarbons from the carbon dioxide obtained in the carbondioxide separation step and the hydrogen obtained from the liquid ionexchange media.
 19. The method of claim 15, wherein the carbon dioxideis separated from the acidified seawater by vacuum stripping.
 20. Themethod of claim 16, wherein the hydrogen is separated from the liquidion exchange media by vacuum stripping.